What is the most relevant condition indicated by symptoms of dizziness upon standing, fatigue, and hyperpigmentation?

The symptoms of dizziness upon standing, fatigue, and hyperpigmentation are most closely associated with Addison's disease, which is an adrenal insufficiency condition. In Addison's disease, the adrenal glands do not produce sufficient amounts of cortisol and, in some cases, aldosterone. This deficiency leads to symptoms such as fatigue due to chronic low energy levels and can cause postural hypotension, which explains the dizziness that occurs upon standing.

Additionally, hyperpigmentation, or darkening of the skin, is a unique and significant indicator of Addison's disease. This occurs due to increased levels of adrenocorticotropic hormone (ACTH) that stimulate melanin production, particularly when cortisol is low. Such hyperpigmentation is often observed in areas of the skin that experience friction or pressure.

While thyroid disease can cause fatigue and dizziness, it does not typically present with postural changes and hyperpigmentation. Similarly, hypertension and anemia may contribute to fatigue and dizziness but do not explain the hyperpigmentation, making Addison's disease the most relevant condition reflected by all three symptoms.